Condition (/;)

patt/;test
is a pattern which matches only if the evaluation of test yields True.

lhs:>rhs/;test
represents a rule which applies only if the evaluation of test yields True.

lhs:=rhs/;test
is a definition to be used only if test yields True.

lhs:=Module[{vars}, rhs/;test] allows local variables to be shared between test and rhs. You can use the same construction with Block and With. »

Make a definition with the condition that x should be positive:

Replace all elements which satisfy the condition of being negative:

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